IS defeated in its Syrian capital Raqqa

US-backed militias in Syria have declared victory over Islamic State in its capital Raqqa, raising flags over the last jihadist footholds after a four-month battle.

The fighting was over and the alliance of Kurdish and Arab militias was clearing the city's stadium of mines and any remaining militants, said Rojda Felat, commander of the Raqqa campaign for the Syrian Democratic Forces (SDF).

A formal declaration of victory in Raqqa will soon be made, once the city has been cleared of mines and any possible Islamic State sleeper cells, said SDF spokesman Talal Selo.

In Washington, the US military said that about 90 per cent of Raqqa had been retaken from Islamic State but it expected the SDF to face pockets of resistance.

The fall of Raqqa, where Islamic State staged euphoric parades after its string of lightning victories in 2014, is a potent symbol of the jihadist movement's collapsing fortunes.

Islamic State has lost much of its territory in Syria and Iraq this year, including its most prized possession, Mosul. In Syria, it has been forced back into a strip of the Euphrates valley and surrounding desert.

The SDF, backed by a US-led international alliance, has been fighting since June to take the city which Islamic State used to plan attacks abroad.

A Reuters witness said militia fighters celebrated in the streets, chanting slogans from their vehicles.

The fighters and commanders clasped their arms round each other, smiling, in a battle-scarred landscape of rubble and ruined buildings around the main square.

The flags in the stadium and others waved in the city streets were of the SDF, its strongest militia the Kurdish YPG, and the YPG's female counterpart, the YPJ.

Fighters hauled down the black flag of Islamic State, the last still flying over the city, from the National Hospital near the stadium.

"We do still know there are still IEDs (improvised explosive devices) and booby traps in and amongst the areas that ISIS once held, so the SDF will continue to clear deliberately through areas," said Colonel Ryan Dillon, a spokesman for the coalition.

In a sign that the four-month battle for Raqqa had been in its last stages, Dillon said there were no coalition air strikes there on Monday.

Speaking with reporters in Washington later on Monday via video conference, Dillon said about 100 Islamic State fighters still remained in Raqqa.